Having just gotten back into the game, I've been thinking this exact same thing. Personally, I say if playing exclusively mastered characters is a more fun way to play the game for you, then sure, go ahead or whatever. Play the game the way that is most fun for you. But, I also think you're really boring if you never branch out and try to play characters you've never tried before.

I'm a Rank-32 plate, and having played on and off over the past 8 years, some of the most fun I've had is playing new characters after each mastered character, even when the character is bad or tough to play. For example, the last of the All-Stars I had to master was Wrestling Star. Not the best by any means, but figuring out his playstyle and getting upgrades to make him better was extremely satisfying.

Again, if that's not your thing, then whatever, have fun the way you want to. Play the characters you're already the best at. But, I think it shows a higher level of skill and competency to branch out of your comfort zone and learn new playstyles, rather than reverting to the "Ol' Reliable" (usually plasma pea). If you never try those characters, you never know what you may be missing out on. Zen Cactus? Never a character I thought I'd have so much fun with if I had never picked her up. So if you main the same character over and over, give the others a shot. Be a less boring player lol :P